Primary and Secondary Oil Recovery From Different Wettability Rocks by Countercurrent Diffusion and Spontaneous Imbibition

Abstract

Abstract This study investigates optimum matrix oil recovery strategies in naturally fractured reservoirs (NFR) for different wettabilities and rock types. We compare the recovery efficiencies of two cases: (a) the primary counter-current spontaneous imbibition followed by the diffusion of a miscible phase (secondary recovery) and (b) primary diffusion of miscible fluid without pre-flush of matrix by the spontaneous imbibition. For these recovery strategies, the effects of the matrix shape factor, matrix wettability, and type of miscible displacing phase on the rate of recovery and development of residual oil saturation were clarified experimentally. Cylindrical Berea sandstone and Indiana limestone samples with different shape factors were obtained by cutting the plugs 1, 2.5, and 5 cm in diameter and 2.5, 5, and 10 cm in length. All sides were coated with epoxy except one end. Static imbibition experiments were conducted on vertically situated samples where the matrix-fracture interaction took place upward direction. Mineral oil and crude oil were used as oleic phases. Brine was selected as aqueous phases for the primary spontaneous imbibition recovery. For primary and secondary miscible displacement experiments n-heptane was used as solvent. Wettability of water-wet Berea sandstone samples was altered to weakly water-wet to observe its effects on the dynamics of spontaneous counter-current imbibition and diffusion. Parametric analyses were performed for the appraisal of secondary and tertiary recovery potential of naturally fractured reservoirs by immiscible and miscible fluid injections. The optimal recovery strategies (recovery rate, recovery time and ultimate recovery) for different rock properties were identified and classified. In water-wet cases, starting the recovery with capillary imbibition followed by diffusion was found the optimal way, i.e. both effective and efficient. For limestone or aged sandstone samples, starting the recovery by diffusion yielded a faster recovery rate and higher ultimate recovery.